Selective Recognition of Configurational Substates of Zinc Cyclam by Carboxylates: Implications for the Design and Mechanism of Action of Anti‐HIV Agents

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Abstract
The interaction of metal cyclams with carboxylate groups is thought to play an important role in their binding to the CXCR4 chemokine receptor and in their anti‐HIV activity. Here we report the synthesis of acetate, phthalate, perchlorate and chloride complexes of ZnII cyclam (1,4,8,11‐tetraazacyclotetradecane). The X‐ray crystal structures of [Zn(cyclam)(phthalate)]n(CH3OH)2n and [Zn(cyclam)(H2O)2](OAc)2 contain octahedral ZnII centres. Phthalate acts as a bridging ligand in the former complex, binding through monodentate carboxylate groups, and giving rise to infinite chains in the lattice together with extensive hydrogen bonding between carboxylate donor oxygen atoms and amine and methanol acceptor atoms. The uncoordinated acetate groups and the aqua ligand in the acetate complex are also involved in a rich network of hydrogen bonds and this may account for the unusually long ZnO distance (2.27 Å). In both crystalline complexes, the macrocycle adopts the trans‐III (S,S,R,R) configuration. 1D 1H NMR spectra of all four complexes have been fully assigned by a combination of 2D [1H, 1H] COSY and TOCSY, and [1H, 13C] and [1 H, 15N] HSQC NMR data. In aqueous solution, the stable trans‐III configuration found in the solid‐state equilibrates slowly (hours at 298 K) with trans‐I (R,S,R,S) and cis‐V (R,R,R,R) configurations. The trans‐III configuration is predominant in aqueous solution for both the chloride and perchlorate complexes, but for the acetate and phthalate complexes, the cis‐V configuration dominates. Carboxylate groups appear to stabilize the cis‐V configuration in solution through ZnII coordination and hydrogen bonding. Titration of the chloride ZnII–cyclam complex with acetate confirmed that carboxylates strongly induce formation of the cis‐V configuration. This implies that carboxylates can exert a strong influence over configurational selectivity. Cyclam NH hydrogen bonding is prevalent both in the solid state and in solution, and is relevant to the anti‐HIV activity of ZnII and other metal cyclam complexes and to their ability to recognize the CXCR4 transmembrane co‐receptor.